The invention relates to a method of synchronizing pulse generators in the transmission of digital signals that contain a clock pulse, hereinafter an information pulse, and periodically at least one sync word. From the digital signals a first signal pulse that is phase-locked with respect to these signals is generated. Then, from this first signal pulse, by means of a phase-control loop containing a phase comparator and a voltage-controlled oscillator connected with it, a second signal pulse is generated. In addition, the generation of the first signal pulse and the reception of the sync word are monitored. The transmission of digital signals requires, at specified spatial intervals or at digital interfaces, a regeneration of the signal pulses in terms of both amplitude and time. For time regeneration in this case, a pulse signal is required which corresponds in frequency and phase to the pulse signal used by the sender. For this purpose, on the assumption of continuous signal transmission, the required pulse signal is either transmitted parallel to the digital signal on a separate line to the receiving station, or else the digital signal is coded in such a manner that it includes an information pulse. In this case, the information pulse can consist of energy components at a frequency corresponding to the pulse frequency of the digital signals; energy components can also operate at, for example, twice the pulse frequency, with a signal pulse being generated from them at the receiving end. The generation of the signal pulse can be accomplished by means of an oscillating circuit which is tuned to the corresponding frequency; a pull-in pulse oscillator contained in a phase-control loop can also be used. In this case, there are fed to a phase comparator contained in the phase-control loop, on the one hand, a first signal pulse generated from the transmitted digital signals and, on the other hand, the pulse signal that is generated locally. The local pulse oscillator is then pulled in by means of a voltage corresponding to the phase difference of the two signals. The locally generated signal pulse is used for the time regeneration of the digital signals so that the regenerated signals are virtually jitter-free, and the overall transmission of the digital signals is largely free of any interference. As a result of the small band width of the phase-control loop, the synchronization takes a long time. This is especially true when a larger number of intermediate regenerators is used within the transmission line. It is true that even if a pull-in pulse oscillator is not utilized the first signal pulse, which is obtained from the transmitted signals, can be used for the time regeneration, so that the synchronization time is short. However, the first signal pulse--and, as a result, the regenerated signals as well--will exhibit a severe phase jitter, which can accumulate over the line and lead to interference.
The local generation of signal pulse for regeneration by means of a pull-in oscillator is frequently used, because the transmission lines for digital signals are usually constantly in operation, and if the rate of error is very low, it is very seldom necessary to synchronize again after the initial synchronization has taken place. However, difficulties can occur when the digital transmission line is exposed more frequently to interference or glitches or when the time for resynchronization plays a significant part in the overall operation. This is true in the case of addressless monitoring systems for digital long-distance lines; a monitoring system of this kind is described in published U.S. Pat. No. 4,406,919. In this monitoring system, a telemetry signal is transmitted simultaneously over the transmission cable for the digital information signal, in which case an additional telemetering device with a telemetry signal regenerator and a telemetry signal sender is assigned to each line terminal and each intermediate regenerator, for the digital information signal. The transmission of the telemetry signal begins when the telemetering device in the sending line terminal sends out cyclically an initial telegram followed by a final character. In interference-free operation, each telemetering device in the subsequent intermediate regenerators regenerates every telegram received with its final character, and in addition, adds to the end of the last telemetric data received its own telegram with a final character. In this manner a chain of telegrams is created on the way from one telemetering unit to the next, which, in addition to the telegram from the sending line terminal, contains a telegram from every telemetering unit on the monitoring line. The interpretation of the telegrams takes place in a terminating position-finder.
With a view to reporting cases of interference, every telemetering unit is designed in such a manner that when there is a cessation in the reception of telegrams, the telemetering unit, after a specified waiting period, automatically begins to send out cyclically its own telegram followed by the final characters. However, the sending stops immediately, as soon as telegrams start to be received again. In order to regenerate the digital telegrams and to control sending out its own telegram, each telemetering unit contains a pull-in pulse oscillator, which in interference-free operation is synchronized to the pulse of the telemetering unit in the sending line terminal.
In the event of interference or a glitch--for example, if there is a breakdown in an intermediate regenerator--after the expiration of the waiting period, all the telemetering units start to send out their own telegrams. By this means, however, all the telemetering units up to the one after the point of interference again start to receive telegrams, stop sending out their own telegrams and are synchronized to the pulse oscillator of their predecessor. But since this can also be happening at the same time to the predecessor, the result is a series of synchronizing operations, until the individual telemetering units have ultimately been synchronized along the series to the pulse of the first telemetering unit after the point of interference. Accordingly, the time for the synchronization of the remainder of the line amounts to the sum of one waiting period and the number of telemetering units in the remainder of the line, multiplied by the cycle time, which, in practice, is somewhat larger than the time it takes to synchronize an individual pulse oscillator.
In the least favorable situation the interference takes place in the connection to the sending line terminal, so that, for example, on a transmission line with 312 intermediate regenerators with a cycle time of 6.8 seconds and a waiting time of 14 seconds, the time to synchronize the remainder of the line is about 35 minutes. Not until then is the position-finder in a position, by comparing the number of telegrams for the remainder of the line with the predetermined standard, to specify the location of the interference and to give the appropriate alarm. As a result, however, a monitoring system of this kind would not be feasible for digital long-distance systems with high bit rates.